1. Field of the Invention
The instant invention relates to methods and apparatus for well fluid level control and more particularly to such methods and apparatus in which the fluid level in a well is sensed by initiating an acoustic pulse at the top of the well and detecting the pulse reflected from the fluid surface.
2. Background of the Invention
In most oil and gas wells, especially toward the end of the producing life of the well, it is necessary to pump well fluids to the surface of the well because the pressure in the formation from which the well is producing is insufficient to force well fluids to the surface.
Pumps on oil wells typically comprise either a downhole submersible electric pump or a beam pumping unit which raises and lowers a string of sucker rods in the well. Regardless of the pump type, well production can be optimized and equipment wear minimized by on and off cycling of the pump for appropriate intervals. It is desirable to control the pump by switching it on when the fluid column rises above a preselected level and switching it off when it falls below a preselected level. As fluid flows from the formation into the well bore, the fluid level in the well rises. Typically, at some point as the height of the fluid column increases, fluid influx into the well is reduced. If the pump is not energized before the decrease in well influx, production is not occurring at an optimum rate. At the other extreme, if the pump turns on too soon, the pump is cycling on and off at a fairly rapid rate and may be drawing the fluid level down below the pump input thereby permitting gas to enter the pump. Such a condition increases pump wear and causes hammering of sucker rods, flexing in tubing strings and other undesirable effects.
There exists a number of prior art pump controllers for switching the pump on when the fluid level in the well rises above a first preselected level and switching it off when the fluid level falls below a second preselected level below the first preselected level. Such prior art units typically include means for initiating an acoustic shot or pulse at the surface of the well which travels down the well bore and is reflected from the fluid surface in the well back to the surface. The reflected pulse is detected at the surface and the time between initiation of the shot and detecting the reflected pulse is measured thus providing an indication of the fluid level when the velocity of the pulse is known. Some of the prior art controllers includes means for selecting high and low fluid levels at which the pump is automatically turned on and off, respectively.
Such prior art controllers suffer from several disadvantages.
One such prior art controller produces a rough estimate of fluid level depth by starting a timer simultaneously with a signal which opens a solenoid in order to release pressurized gas thereby generating an acoustic pulse in the well. The pulse travels down the well toward the well fluid and is reflected therefrom back to the surface. A transducer in the well adjacent the surface detects pressure and converts the same to an electrical signal. A threshold circuit is set to stop the timer when the monitored electrical signal rises above a preselected level. The time so generated is converted to a depth by multiplying the same by the estimated velocity of the acoustic pulse in the well.
The above-described prior art system suffers from a number of drawbacks. First, the surface of the fluid column in the well is typically not the only source of reflected acoustic pulses which are detected by the transducer. Any change in the cross-sectional area of the well bore causes a pressure change to be reflected back to the surface. For example, in the case in which a production tubing string is suspended in the well bore, the pulse is transmitted in the annulus between the tubing string and the casing. Each set of tubing collars, at which adjacent joints of production tubing are connected, decreases the cross-sectional area of the well annulus and thus reflects acoustic energy back toward the surface. Well equipment such as centralizers, tubing anchors and the like also decrease annulus area thus generating reflected pulses which are detected at the surface. In addition, fluid influx into the well at perforation zones reflects acoustic energy back toward the surface. In a similar fashion, when annulus cross-sectional area increases, due to, e.g., an opening in the casing, a pressure reduction is detected at the surface. Thus, it can be seen that pulses generated by a number of sources are detected at the surface. In addition to pulses generated by such area changes there are often significant levels of mechanical and/or electrical noise which is detected by the transducer at the top of the well.
The prior art fluid depth controllers of the type described often mistake detected pulses which are produced by, e.g., well equipment, or which appear as a result of electrical or mechanical noise as the pulse reflected from the fluid surface. Any pulse which has a magnitude greater than that selected in the threshold circuit stops the counter. Pulses generated by well anomalies such as equipment or producing zones or by noise can be mistaken for the pulse reflected from the well fluid thus producing a depth indication which is incorrect.
Another problem relating to such prior art controllers is that estimates of the acoustic pulse velocity are typically crude thus introducing additional error into depth calculations even if the pulse travel time should happen to be reasonably correct. In addition, once a velocity estimate is entered, it remains fixed. Since velocity can vary from shot to shot, error is introduced. One prior art technique for estimating velocity is simply to assume a given velocity. However, velocities can vary significantly depending upon the pressure, temperature, and composition of the gas column in the well above the fluid. One prior art controller similar to that described above may be calibrated in a crude manner which attempts to take such velocity changes into account. Successive shots are fired to locate a pair of adjacent tubing collars. The difference in the depth readings for the adjacent collars is the tubing joint length and provides a means of calibration based on the known distance between the collars. Since velocity is calculated only over the length of a single tubing joint, small percentage errors, in the range, of e.g., 1 to 2 percent, show up as errors of 150 to 300 feet in a fluid depth indication for a well in the 15,000 feet range. Such errors are totally unacceptable if it is desired to have a low set point for the fluid level of, e.g., 50 feet above the pump. Such a set point could not be set on the prior art system without any assurance that the pump would not be energized until the fluid level reached the pump inlet thereby entraining gas into the pump.
All of the prior art systems process the monitored data in real time. It would be desirable to store the data and analyze the same to extract information relating to the relative heights of the pulse peaks, the shape of the peaks, etc. in order to extract useful information for determining the fluid level depth.